The goal of this proposal is to understand the function of the poliovirus receptor (PVR) in one type of blood cell, the monocyte. Despite the control of poliomyelitis in this country through the use of safe vaccines against poliovirus (PV; an enteric picornavirus), polio remains a potential threat. The emergence of post-polio syndrome (PPS) has focused attention on the gaps in our knowledge of the mechanisms of PV infection, replication and pathogenesis. Despite the identification of PVR in 1989, understanding the role of pVR in pathogenic mechanisms has been disappointingly slow. Certain long-standing issues: the site of primary viral replication, mechanism of entry into the CNS, location of extraneural sites of replication have not been elucidated. In order to study both the normal and pathogenic function of PVR, I determined the levels of expression in human blood cells. I found that PVR is expressed on the majority of human monocytes. Furthermore, I showed that PBMCs support PV replication. Our recent efforts have been directed towards demonstrating that PVR-positive cells (monocytes) support replication and characterizing the replication cycle. In preliminary studies presented here, 1 show that; (1) adherent cells alone support PV replication; (2) the majority of PBMCs take up PV and other blood cell types do not/ (3) gamma-IFN increases PVR cell surface levels about 5-fold; (4) a very low percentage of PVR+ PBMCs produce infectious virus; (6) attenuated PV-type 1 and PV-type 2 replicate better in MPs than their virulent counterparts and (7) we have started to express PVR in yeast. We have also shown that D171, an anti-PVR mAb blocks PV replication in MPs. PVR has two distinct functions: it mediates poliomyelitis and it has an unknown normal function. Our studies of PVR have led to a heuristic model to explain the mechanism of PVR function in disease and normal states. The role of viral receptors may be more dynamic that has been thought in the past. During the proposed five year funding period, I will cary out the following specific aims in order to test aspects of the model: (1) Complete the characterization of poliovirus replication in monocytes, (II) generate reciprocal recombinants between the poliovirus receptor (PVR) and human insulin receptor (PVR) and use these to develop functional assays for the normal function of pVR and (III) generate recombinant, soluble, tagged forms of the poliovirus receptor for use in assays to detect the putative PVR ligand. The long term aim of this research is to demonstrate that PV replication in MPs may contribute to pathogenesis by PV and may resolve some of the unanswered questions of PV pathogenesis. Furthermore, these studies may be pertinent to pathogenic mechanisms by other picornaviruses or other macrophage-topic viruses.